Fix build errors: FTPS AsyncRead bridge, TLS connector type, unsafe env::set_var, dead code

Verified with cargo check + cargo run --example printer_polymorphism.

src/printer/mod.rs

@@ -54,7 +54,12 @@ pub trait Printer {
     /// REQUIRED, no default body: every printer vendor speaks a different
     /// protocol, so there's nothing sensible to share here. Each `impl`
     /// below provides a completely different body — that's the "override".
-    async fn connect(&mut self) -> Result<(), PrinterError>;
+    ///
+    /// Spelled as `-> impl Future<..> + Send` rather than plain `async fn`
+    /// so the resulting future is `Send` and can be `.await`ed inside a
+    /// `tokio::spawn`ed task; each `impl` below still just writes a normal
+    /// `async fn` body, which satisfies this signature automatically.
+    fn connect(&mut self) -> impl std::future::Future<Output = Result<(), PrinterError>> + Send;
 
     /// OPTIONAL: a default body every printer gets for free unless it
     /// chooses to provide its own. None of ours do, so all three vendors